The manufacture of surface coatings involves the mixing of pigment in a suitable vehicle and dispersing the pigment to a satisfactory fineness, color, and consistency to form a mill base. The vehicle, as the term is used in the coatings industry, includes film-forming materials, resins, oils, thinners, dispersants, and driers, as well as mixtures of these components. The mill base, with pigment suitably dispersed, can then be incorporated into an appropriate coating formulation.
In preparing the mill base, the pigment is dispersed to a satisfactory fineness by breaking up all agglomerates of pigment particles and thoroughly wetting each particle with the liquid vehicle. This dispersion of pigment with the liquid vehicle has been achieved before by the use of various apparatus such as roller mills and media mills, including ball mills, pebble mills, and sand mills. Media mills are essentially high-shear mixers wherein rotation of the mill or of stirring elements relative to the mill in combination with the presence of media such as steel balls, pebbles, or sand imparts shearing action onto the pigment/vehicle mixture to accomplish dispersion. A process for dispersing pigment by agitating the pigment and vehicle in a suitable apparatus with 20-40 mesh sand is described in U.S. Pat. No. 2,581,414, issued Jan. 8, 1952 to Hochberg.
Media mills generally operate in a continuous fashion such as a continuous stirred tank reactor. In continuous operation, pigment/vehicle premix is added while simultaneously removing material that has been subjected to the shearing action of the mill. As with continuous stirred tank reactors, the distribution of dwell time under shear for proportions of particles can be broad and disparate, necessitating several passes of the pigment/vehicle premix through the media mill to insure adequate dispersion. A typical media mill unit may have a vessel size of perhaps 30 gals. When finishing a run, or changing to a different color mill base, it is necessary to clean out the vessel. This can result in as much as 12 gallons of material being washed out and thrown away.